Products for which a semiconductor is applied are very important products necessary for a variety of industries and a semiconductor chip, one of the most typical products among them is, for example, produced by slicing a single crystal silicon into a given thickness to manufacture a silicon wafer, and then forming various circuits thereon.
To form a variety of such circuits and the like, it is required to carry out steps of applying a photosensitive resin to a silicon wafer, exposing and developing the resin, and then subjecting the resulting resin to post curing treatment or to sputtering treatment to form a conductor layer. For these steps, the silicon wafer is required to be heated.
As such a kind of a heater for heating a semiconductor wafer such as a silicon wafer used in the condition of setting the semiconductor wafer thereon, conventionally those equipped with resistance heating elements such as electric resistors on the bottom face side of a substrate comprising aluminum are employed most, however the substrate comprising aluminum has a thickness of about 15 mm and therefore is heavy and bulky and not necessarily easy to be handled and insufficient in temperature controllability in terms of the temperature-following property to the electric current application to make even heating of a semiconductor wafer difficult.
In the publication of JP Kokai Hei 11-40330, there is disclosed a ceramic heater composed of a substrate of nitride ceramics or carbide ceramics with a high thermal conductivity and strength and heating elements formed by sintering a metal particle on the surface of a plate-like body (a ceramic substrate) comprising these ceramics.
Further, as for a heater to be employed for such a semiconductor producing device, the surface of the resistance heating elements thereof is easy to be affected by light and heat, treatment gases and the like when it is used as the semiconductor producing device, thus the resistance heating elements are required to have durability to oxidation on the surface.
Therefore, the inventors of the present invention have made investigations aiming to form a resistance heating element excellent in durability and consequently found that formation of an insulating covering on the resistance heating element formed on a ceramic substrate makes a ceramic heater excellent in durability, for example, anti-oxidation property and the like. However, the insulating covering may work also as a heat insulator for the resistance heating element, so that at the time of cooling after the ceramic heater is heated, quick cooling sometimes becomes impossible.
Further, as a method for forming the resistance heating element at the time of manufacture of such a ceramic heater, conventionally, the following methods have been employed; a method for forming the resistance heating element by a coating process such as screen printing; a method for forming the resistance heating element by a physical deposition method such as sputtering and a plating method after producing a ceramic substrate with a given shape.
In the case of a method for forming the resistance heating element using a coating method after producing a ceramic substrate with a given shape, a conductor containing paste layer in a heating element pattern is formed and successively heating and firing is performed to form the resistance heating element.
However, although the resistance heating element can be formed at a relatively low cost, such methods have a problem that the resistance heating element with a precise pattern can not be formed easily since trifling mistakes at the time of printing result in short-circuit in the case of producing a precise pattern. The above-mentioned method has another problem that the printing thickness is not even and subsequently, the resistivity becomes uneven.
Further, in the case of a method for forming the resistance heating element using a physical deposition method such as a sputtering and a plating method, after producing a ceramic substrate with a given shape, a metal layer is formed in a given area of the ceramic substrate by these methods and successively etching resist is formed so as to cover the portions on heating element patterns and then etching treatment is performed to form the resistance heating element in the given patterns, or at first the portions other than the heating element patterns are covered with resin and the like and then the above-mentioned treatment is carried out to form the resistance heating element in the given patterns on the surface of the ceramic substrate by one time treatment.
However, although this sputtering or the plating method and the like is capable of forming precise patterns, the method has a problem that etching resist or plating resist has to be formed on the ceramic substrate surface by a photolithographic technique in order to form the resistance heating element with given patterns, resulting in high cost.
As a method for solving these problems, a method has been employed which has an advantage that precise resistance heating element patterns can be formed at a relatively low cost, that is: a method comprising steps of forming a conductor layer in a strip-shaped or a ring-shaped with a given width and then removing the portions other than the heating element patterns using a laser beam irradiating equipment and the like to form precise heating element patterns; or a method including the steps of forming the resistance heating element by the above-mentioned method and successively irradiating laser beam to adjust the thickness of the resistance heating element or to remove some portion of the resistance heating element so as to precisely adjust the resistant value.
However, by a conventional screen printing and the like, the surface of the resistance heating element or the conductor layer is smooth and at the time of performing trimming by laser beam irradiation, in some cases, the laser beam is reflected at the surface of the resistance heating element. Consequently, it becomes impossible to perform trimming the resistance heating element or the conductor layer as designed, resulting in unevenness of the depth and the width.